Здравствуйте, Serginio1, Вы писали:

S>На yield построен Linq
S>Непонятно?

Как тебе такой linq без yield и даже без IEnumerable?

static void Main()
{
    var n =
        from c in 1.ToMaybe()
        from s in "2".ToMaybe()
        from x in 2.ToMaybe()
        select s + c + x;

    Console.WriteLine(n);
}

abstract class Maybe<T>
{
    public class Nothing : Maybe<T> {}
    public class Just : Maybe<T>
    {
        public T Value;
    }

    public override string ToString() => this switch
    {
        Just j => j.Value.ToString(),
        _ => "Nothing"
    };
}

static class Extensions
{
    public static Maybe<T> ToMaybe<T>(this T value)
    {
        return value == null ? (Maybe<T>)new Maybe<T>.Nothing() : new Maybe<T>.Just { Value = value };
    }

    public static Maybe<T2> SelectMany<T1,T2>(
        this Maybe<T1> source, Func<T1,Maybe<T2>> selector)
    {
        if (source is Maybe<T1>.Just j) return selector(j.Value);
        return new Maybe<T2>.Nothing();
    }

    public static Maybe<T3> SelectMany<T1,T2,T3>(
        this Maybe<T1> source, Func<T1,Maybe<T2>> collectionSelector, Func<T1,T2,T3> resultSelector)
    {
        return source.SelectMany(x => collectionSelector(x).SelectMany(y => resultSelector(x, y).ToMaybe()));
    }
}